The present invention relates to a photographing apparatus for an endoscope and, more particularly, to an automatic exposure controller such for a photographing apparatus.
In a photographing apparatus for an endoscope comprising a camera unit and a light source unit mounted to the endoscope, a photosensitive element or photodetector is disposed inside the eyepiece part of the endoscope or the camera unit for measuring the luminosity of light incident on the film inside the camera unit. According to the photoelectric signal from the photosensitive element, the automatic exposure control unit of the photographing apparatus calculates the luminosity of the light incident on the film. When this calculated amount of light reaches the proper exposure value, supply of the photographing light emitted from the light source unit is interrupted and the film is exposed to the optimal amount of light.
The photosensitive element has photoelectric conversion characteristics perculiar to itself; therefore the precise amount of light incident on the film may not be measured by the photocurrent generated by the photosensitive element directly as an object luminance signal corresponding to the light intensity of the film surface. This is because the commercially available photosensitive elements do not uniformly generate photoelectric signals of the same level for an object of a given luminosity. In order to solve these problems, a conventional photographing apparatus for an endoscope has been proposed in which a resistor for adjusting sensitivity is connected in parallel with the photosensitive element, and the resistance of this resistor is suitably adjusted so that the variations in the photoelectric conversion characteristics of the photosensitive element may be compensated and the correct luminosity information may be obtained. When light of a certain luminosity is projected on the photosensitive element, the resistance of the sensitivity-adjusting resistor is so adjusted that the proper voltage is induced across the two ends of the sensitivity-adjusting resistor by the photocurrent generated by the photosensitive element, and this voltage may then be supplied to the automatic exposure apparatus as an object luminance signal.
However, as has been described, when the resistor is connected in parallel with the photosensitive element, the amount of light projected on the photosensitive element becomes great. When the voltage generated across the resistor reaches a predetermined level, the current flowing through the photosensitive element is saturated, in which event the voltage of the resistor may no longer correspond directly to the illumination of the film surface, resulting in overexposure.
It is an object of the present invention to provide a photographing apparatus for an endoscope which is capable of constantly providing a proper exposure.